When you purchase My Media Center the license is tied to the account that you are logged into on your Android device. You are able to install My Media Center for Android on as many devices that you would like as long as your account that is licensed is added to the device. New in Android 4.2 is the ability to have multiple users per device. You can still follow these same steps to get this working:  

1.  Log into one of the secondary user profiles(Android 4.2+) or other device
2.  Start Google play
3.  Select "menu" (3 dots on top of each other on some devices, or the menu button)
4.  Select accounts
5.  Select "add account"
6.  Enter the credentials for the "primary" account that has the purchase associated with it.
7. Go to "all apps" and find My Media Center and you should now be able to install it install it